Frequency modulation indicator



March 17, 1942.

Filed Oct. 26, 1940 K .4 l0 l2 CONVERTEB 3a 6 T ii. s y 1054K 70 VOLTAGE u /?[6' 77/75? INDICATOR v v I I E w AL 4 "/OSCILLATOR INVENTOR WALTER M4 Patented Mar. 17, 1942 o v I Q t UNITED STATES PATENT OFFICE 2,276,672 FREQUENCY MODULATION INDICATOR Walter van B. Roberts, Princeton, N. J., assig-nor to Radio Corporation of America, a corporation of Delaware ApplicationOctober 26, 1949, Serial No. 362,908 Claims. (01. 179-4715) This application concerns a new and improved is equal to the reactance of Q at the mean fremeans for investigating the characteristics of quency. Inductance S is as shown in'series with wave energy of varying frequency. The means of condenser Q. The voltage across Q is added to this application is particularly adapted to use for the voltage produced by the mutual inductance monitoring the output of a frequency modulated 5 M and the polarity of the mutual is such that transmitter, the sum of these two voltages is .zero at the In describing my invention, reference will be means frequency. By using the voltage produced made to the drawing wherein the single figure by M on the rectifier I6, I avoid applying thereillustrates diagrammatically the essential feato any drop produced'by current flowing in L. tures of a wave monitoring means arranged in If the resistance of L could be made low enough, accordance with my invention. winding S could be omitted and the top end of In the drawing, A represents wave pick-up L coupled to the left hand end of C. If this remeans and for purposes of illustration is shown vision was made, L and Q would, of coursepthen as an aerial. The wave energy picked up by A be tuned to the mean frequency of the interis supplied to a wave amplifying, heterodyning, l5 mediate frequency energy supplied by tube 8. s

detecting, and intermediate-frequency amplifying When the frequency of the output of tube 8 means included in unit 2 and unit 4, the latter departs from the mean in either direction, the being designated as a source of local oscillations. amplitude of the voltage impressed on the diode Wave energy of the intermediate frequency is' l6 increases from zero in proportion to the fresupplied to the control grid- 5 of an electron dis quency departure. If the capacity C and the Charge de ce and pp from tube 8 to a resistance R form a time constant which is long tuned circui l h inductance of Which forms compared to the period of the lowest modulation the P y Winding Of a transformer the frequency, but short compared'to variations in Secondary winding of which is the inductance N- program level, the potential of the diode anodes This secondary windingN is ina circuit compris- 4 will be proportional t t maximum fre ins con en r K inductance L, condenser Q and quency deviation, regardless of whether this dea second inductance between which and L a viation happens to be on one Side of the meanmutual inductance-M is developed. The output f u n or the other. Thus a galvanometer of this circuit is coupled by coupling condenser 2 in series with R will indiooito by exceeding C to a diode rectifier 16 across which is connected 3 a certain reading, that ovepmodiflatidn is a very high resistance 20 and a galvanometer 22. mg place In place of a galvanometer u there can, of course, be used the ordinary so-called The high resistance 20 causes the rectifier 16 to act as a peak voltage rectifier. l

In operation, wave energy, the frequency of fizg g connected to voltage which varies, is picked up at A, amplified and oied'lgced lg. sfreiiugncg in 2 and 4, and supplied g g gfig gg g aig fig ig z g f i gg z gggs otegri ofue The tube 8 may be adjusted to operate either g g zg gg ggg g fzi ag zfig gg g as an amplifier or a limiter. In the plate circuit of C to maintain the direct currem e te i of this tube is a circuit l0 tuned to the mean 40 d finite] t no u or 3 s e i: assassi ate $223 fg fig g gg gazfi f 33; the aforesaid choke coil 28 and adjust the resistcouplings and power factors of these two circuits ance to h a Value at its leading effect are preferably so chosen that the current in the on the tuned circuit W111 assist in a s;

' secondary circuit NKLQ is substantially constant the current through L and Q more nearly over the full range of transmitter frequency, 1. e., stantthe intermediate frequency, corresponding to The value of the present arrangement is that it maximum allowable moduhtion, h t is t reads directly in proportion to the-modulation if these couplings are an'anged to pass substan- L: the modulation is symmetrical, while if the modtially uniformally a band of'frequencies includelation is unsymmetrical its reading is op ring the band through jwhieh the wave supplied tional to the maximum instantaneous deviation. by t 3 deviates regardless of the direction thereof, and this Another coil S is coupled to L, with mutual inlatter deviation is what must be k p Within ductance M adjusted so that its mutual reactance predetermined limits in order to avoid overshooting the characteristics of the discriminator at the receiver.

What is claimed is:

1. A modulation indicator for frequency modulated transmission including, means for developing a voltage the amplitude of which is substantially zero at the carrier frequency and which increases substantially in proportion to the difference between the instantaneous frequency of the transmission, and the carrier fre-' quency thereof, a peak rectifier in a circuit including impedance and capacity coupled to said means for rectifying said voltage, an indicator coupled to said rectifier for indicating the rectified voltage, the time constant of the circuit of the rectifier being greater than the longest period of the modulation, but short comparedwith variations in modulation level.

2. In means for analyzing the characteristics of wave energy of varying frequency, a tuned circuit including an inductance and a capacitance in a closed circuit tuned to the mean frequency of said wave energy, means for impressing said wave energy on said tuned circuit, a second circuit including, in series, an inductance and at least a portion of the capacitance'in said first circuit, said last named inductance being coupled to an inductance in said first circuit, peak rectifying means coupled with said second circuit, and current indicating means coupled with capacitance of the second circuit at the mean frequency of said impressed wave energy, rectifying means coupled with said third circuit, and current indicating means coupled with said rectifying means.

4. Inmeans for analyzing the characteristics of wave energy of varying frequency, a tuned circuit comprising an inductance and a capacitance in a closed circuit'tuned to the mean frequency of said wave energy, means for impressing said wave energy on said tuned circuit, a second circuit'including, in series, an inductance and at least a portion of the capacitance in said first circuit, said last named inductance being coupled to an inductance in said first circuit by a coupling such that the total mutualimpedance between said first and second circuits is sub stantially zero at the mean frequency, peak rectifying means coupled with said second circuit,

and current indicating means coupled with said rectifying means.

5. In means for analyzing the characteristics of wave energy of varying frequency, a peak voltage rectifier coupled to direct current indicating means, a closed circuit including'an inductance and a capacity in series, means for impressing wave energy of varying frequency to be analyzed on said capacity and inductance, said series circuit being series tuned to'-the mean frequency of the impressed wave energy whereby voltage of lowamplitude is developed across said series inductance and capacity when said impressed wave energy is of said means frequency and the voltage developed across said series inductance and capacity increases substantially proportional to deviations of the frequency of the impressed wave energy' in either direction from said mean frequency, and means coupling said rectifier to said inductance and capacity. to transfer voltage developed across said inductance and capacity to said rectifier. I

WALTER VAN B. ROBERTS. 

